Pools are groups of posts with a common theme. They are similar to favorites with three important differences: public pools allow anyone to add or remove from them, you can create multiple pools, and posts in a pool can be ordered. This makes pools ideal for subjective tags, or for posts that are part of a series (as is the case in manga).
The interface for adding and removing pools resembles the interface for favorites. You can click on Add to Pool from the post's page. You'll be redirected to a page where you can select the pool.
If you're importing several posts into a pool, this process can become tedious. You can instead click on the Import link at the bottom of the pool's page. This allows you to execute a post search using any tag combination you would normally use. Remove any posts that are irrelevant to the pool, then finish the import process.
Pools can be private or public. A private pool means you are the only person who can add or remove from it. In contrast, public pools can be updated by anyone, even anonymous users.
To remove a post from a pool, go to the pool's page and select the Delete Mode checkbox. Then click on the posts you want to delete. This works similarly to how posts are deleted from favorites.